This invention relates to internal combustion engines, and more particularly to a centrifugally responsive vacuum release mechanism.
In a normal four stroke pull-start engine, a starting event moves the engine through one or more engine cycles to start the engine. The starting event may involve a person pulling a pull cord, or an electric starter, rotating the engine. The engine cycle has four strokes: the intake stroke, the compression stroke, the expansion stroke, and the exhaust stroke.
During normal engine operation, an air/fuel mixture is ignited just before the expansion stroke to power the engine and move the engine through the engine cycle. During pull starting, the operator must exert enough force to overcome the resistive force of the compressed air in the combustion chamber during the combustion stroke. The additional force required to compress the air increases the torque on the cord and makes the engine more difficult to start.
A compression release mechanism may be used to release pressure in the combustion chamber during the compression stroke, which reduces the torque and resistive force on the cord. The reduced torque makes the engine easier to start because the operator does not have to exert as large of a force on the pull cord to move the engine through the cycle. Typically, a compression release mechanism slightly unseats an engine valve to vent the combustion chamber during the compression stroke while the engine is rotating at starting speeds. The compression release mechanism generally disengages at or before the engine reaches normal operating speeds.
The object of the compression release mechanism is to reduce the torque on the cord by releasing the pressure in the combustion chamber during the compression stroke. Since the combustion chamber is relatively airtight when the engine valves are closed, the release of pressure during the compression stroke creates a partial vacuum in the combustion chamber for the expansion stroke. When starting an engine having a compression release mechanism, the operator must exert enough force on the pull cord during the expansion stroke to pull the piston against the partial vacuum in the combustion chamber. The additional force required to overcome the partial vacuum during the expansion stroke creates a torque and the resistive force on the cord, and makes the engine more difficult to start.
A feature of the invention is to reduce the resistive torque of an internal combustion engine during a starting event. The starting event usually involves a person pulling on the pull cord to start the engine, but the starting event could also include an electric starter rotating the engine through the engine cycle to start the engine. The engine comprises a reciprocable piston, a combustion chamber located on a first side of the piston, a crankcase located on a second side of the piston that is opposite the first side, and a cam shaft. The engine has a valve operating system comprising a cam interconnected to the cam shaft, a cam follower capable of contacting the cam, and an engine valve responsive to movement of the cam follower.
The engine also includes a centrifugally-responsive vacuum release member located near the cam. The vacuum release member engages the cam follower at engine starting speeds to unseat the engine valve while the piston is moving toward the crankcase and away from the combustion chamber.
A mechanical vacuum release slightly unseats the engine valve to relieve the vacuum in the combustion chamber during the expansion stroke while the engine is cranking and running at starting speeds. The unseated engine valve relieves the vacuum by permitting air to enter the combustion chamber during the expansion stroke.
The mechanical vacuum release comprises the vacuum release member, the cam follower, and the engine valve. The vacuum release member is centrifugally-responsive and generally disengages at or before the engine reaches normal operating speeds. The vacuum release member is generally in an engaged position when the engine is rotating at engine starting speeds, and in a disengaged position when the engine reaches normal operating speeds. When the engine speed reaches a desired kick-out speed, centrifugal forces enable the vacuum release member to move from the engaged position to the disengaged position.
The vacuum release member of the invention is illustrated in multiple embodiments. In a first embodiment, the vacuum release member is pivotably interconnected with the cam to pivot between an engaged position and a disengaged position. The vacuum release member includes an engaging portion, a flyweight portion, and a bridging portion. The engaging portion has an arc-shaped cam surface that extends beyond the cam in a radial direction, and engages the cam follower when the vacuum release member is in the engaged position. The flyweight portion has sufficient mass to move the cam surface in response to engine speed. The mass of the flyweight portion is preferably greater than the mass of the engaging portion. The U-shaped bridging portion interconnects the engaging portion and the flyweight portion. The vacuum release member is retained within a slot formed in the cam. The slot extends radially inward into the cam, and is partially defined by two side walls and a back surface. The back surface bears load forces imparted on the vacuum release member by the cam follower.
In a second embodiment, the vacuum release member includes a beam and a blocking member. The beam may be cantilevered with a cam surface near the cam, and a bracket at the end of the beam opposite the cam surface. The bracket interconnects the beam to a cam gear. The cam surface engages the cam follower at engine starting speeds. The blocking member is coupled, preferably pivotably, to the cam shaft, and may move between an engaged position and a disengaged position. A tab may project from the blocking member near the coupling between the blocking member and the cam shaft. When the blocking member is in the engaged position, the tab is located between the beam and the cam shaft, and supports the beam against forces exerted by the cam follower. When the blocking member moves to the disengaged position, the tab moves away from its position between the beam and the cam shaft. Without the blocking member supporting the beam, the cam follower deflects the beam, and the cam follower may contact the cam for the entire engine cycle.
In a third embodiment, the vacuum release member and a compression release member are both interconnected to a single yoke that is pivotably coupled to the cam gear. Two separate tabs project outward from the cam shaft. A vacuum tab projects for the vacuum release member, and a compression tab projects for the compression release member. The yoke may pivot between an engaged position and a disengaged position. When the yoke is in the engaged position, the vacuum tab and compression tab both contact the cam follower as the cam gear rotates. Since the vacuum release member and the compression release member are both interconnected to a single yoke, they both pivot to the disengaged position at the same time.
In a fourth embodiment, the vacuum release member and compression release member are also both interconnected to a single U-shaped yoke that is pivotally coupled to the cam gear. The vacuum release member and the compression release member are bulges that project outward from a closed curved end of the yoke, and are substantially planar with the closed curved end. The yoke has curved U-shaped recesses on legs that extend from the curved closed end to an open end. A pin is disposed in the recesses and retains the yoke. The yoke pivots about the pin, and the yoke may pivot between an engaged position and a disengaged position. When the yoke is in the engaged position, the vacuum release member and compression release member both contact the cam follower as the cam gear rotates.